In the art of induction logging of boreholes, in which electromagnetic energy is transmitted into earth formations traversed by the borehole and the formation response to that transmitted energy is analyzed in order to measure the resistivity of the formation fluids, there has long been difficulty associated with the displacement of formation fluid by the drilling fluid. In porous and permeable formations, the pressure in the borehole forces the drilling fluid into a radial zone near the borehole, displacing formation fluids with drilling fluid having a different resistivity. Since the earliest patents pertaining to focussed coil systems for induction logging (U.S. Pat. Nos. 2,582,314 and 2,582,315, issued to H. G. Doll on Jan. 15, 1952) and incorporated herein by reference, the art has attempted to reduce the contribution to the tool response made by that part of the formation invaded by the drilling fluid ("invaded zone") and by formations above and below the region of interest. For example, U.S. Pat. No. 3,067,383, issued to D. R. Tanguy on Dec. 4, 1962 and incorporated herein by reference, discloses a sonde that has been very widely used in the industry and U.S. Pat. No. 2,790,138 issued to A. Poupon on Apr. 23, 1957 discloses the use of a plurality of electrically independent transmitter-receiver pairs arranged symmetrically about the same center point. The response of that tool is obtained by combining the responses of the several electrically independent pairs, these pairs being arranged in such a manner that contributions to the tool response from formation regions lying above or below the outermost coils and from the formation region close to the borehole are reduced.
It has been possible to combine an induction tool with a spontaneous potential measurement or with a pad-type device, but it has not been possible in the prior art to combine in close proximity an induction tool with a tool measuring a different parameter of the formation, such as porosity or radioactivity. In the prior art, separate tools have been stacked vertically, with other tools being located a considerable distance above the induction tool. This arrangement has the considerable disadvantage that the bottom of the well will not be logged by all tools, so that in order to obtain more than one log of a target zone on the same pass through the well, the borehole must be drilled past the target zone.